A complementary metal oxide semiconductor (CMOS) image sensor is a key component of many digital video cameras and other “high tech” devices. The CMOS image sensor is typically comprised of an upper stack that includes one or more layers of color filters and a microlens array, and a lower stack that includes interlevel dielectric layers, interlevel metal layers, and passivation layers that are formed on a substrate. The function of the microlens component is to focus incident light through a light column onto a sensing area (photodiode) at the base of the lower stack. The elementary unit of the image sensor is a pixel which is an addressable area element with intensity and color attributes related in large part to the spectral signal contrast obtained from the photon collection efficiency of the microlens array, light transmission through the color filters, microlenses, and other layers in the imaging path, and the spectral response and efficiency of the photodiode. A pixel converts incident radiation into a quantity of electrical charge that is related to the intensity of illumination. Output signals from a plurality of pixels are used by the image sensor device to generate a picture.
A plurality of pixels forms an array on the substrate wherein pixels sensitive to red light, blue light, or green light are evenly distributed. Ideally, all pixels of a certain color should afford the same output in terms of electrical charge when exposed to the same intensity of incident light. However, process variations during CMOS image sensor fabrication and other factors such as particle defects on the surface of the image sensor cause the output of certain pixels to vary either above or below the desired output range. If a large enough number of the pixels in the array fail to provide an acceptable output signal, the image sensor is rejected. Therefore, pixel arrays are typically tested while still on the chip and before the image sensor is incorporated into a larger device.
Testing is a major cost component of the final image sensor device because of the large number of pixels that must be tested, and the nature of the test which includes both optical and electrical methods. One important test is the so called photon transfer curve (PTC) test where the light sensitivity of an array of pixels in response to incident light is determined. The PTC characterizes the image sensor in many different ways. In order to calculate the read noise, the dynamic range, conversion gain, offset, offset fix pattern noise and the full well, it is necessary to grab frames with different exposure times at a certain illumination. The exposure time or integration time is usually named in number “s” of rows. One row exposure is equivalent to the number of pixels per row multiplied by clock frequency in microseconds. The illumination is chosen in a way such that the sensor reaches saturation (white image, maximum output level) at the longest exposure time.
During the PTC test, a measured amount of broad band light that is highly uniform over the entire exposure field is directed at the pixel array through a point light source. Each pixel has a photo gate (photodiode) at the bottom of its light column that is pre-charged to a certain voltage level. During the time that pixels are exposed, the light photons discharge the photo gate and the intensity of light incident on the photo gate is related to the amount of discharge. The remaining voltage is transferred to a storage node (capacitor) where the voltage stays until the “pixel information” (voltage level) is read out. The read out time is related to exposure time. Normally, the test involves 5 to 10 different illumination levels (different exposure times) and each illumination requires two frames wherein a frame is defined as a certain number of lines (rows) of pixels from an array that consists of “m” rows and “n” columns of pixels. A typical exposure rate of 30 frames per second and the wait times between light intensity changes dictate that the total PTC testing time is about 6 seconds per device. Note that the sequence of illumination, voltage transfer to storage, and read out of data to a frame grabber and ultimately to an image processor can be performed simultaneously for different frames.
A dark current test is also performed for each device. The dark current is the parasitic leakage of the storage node and testing involves resetting the photo gate to a dark state which is equivalent to a condition where there is no illumination of the pixel. Generally, the dark current test requires acquisition of a “dark” frame following each of the 5 to 10 different exposure levels. In other words, to calculate the dark current in nA/cm2, it is typical to have the same frames as for the PTC (fixed illumination, different exposure times). However, it is also necessary to grab one additional frame for each exposure time setting but without illumination. Thus, if 10×2 or 20 frames are needed for the PTC, an additional 10 frames would be required for a typical dark current test. The parasitic leakage may be significant for long read out times (long exposures).
Since common light sources are not programmable and fast switching, it is generally not feasible to vary the illumination (photon flux) during the frame. Moreover, changing the intensity of the light source from one exposure to the next is less accurate than changing the exposure level by varying the time for a fixed illumination. Changing the light intensity will result in intensity differences from one frame to the next of about 1% or less but that variability is enough to produce less accurate calculations than when intensity is kept constant and exposure time is varied. As a result, improvements in PTC and dark current test throughput are limited because of the multiple number of frames required in the standard test method. Therefore, a faster method of testing image sensors and one that has improved accuracy is needed to reduce the cost associated with device fabrication.
In U.S. Pat. No. 6,625,558, a method and apparatus that enable fast testing of light sensing integrated circuits are disclosed. The test involves a low voltage differential signal data transfer link from a test head to an image data interface card in a test signal processor computer. However, the method does not address the lengthy acquisition times necessary for pixel illumination and dark current tests.
A multiphase charge-coupled device is disclosed in U.S. Pat. No. 4,963,952 and has a photosensitive volume bounded by SiO2 layers on the front and back. Dark noise is reduced by applying a different negative bias at the front and back.
An illuminator is described in U.S. Pat. No. 6,737,637 wherein output light from a first integrating sphere is spatially divided and delivered to a plurality of second integrating spheres. The output lights from the second integrating spheres are directed onto active regions of respective image sensors as a means of reducing test time.
An image sensor is described in U.S. Pat. No. 6,326,230 wherein photocharges accumulated in a photoactive region during a pixel integration period are transferred to a sense node during a charge transfer period and are transferred to a power supply node during a third period without passing through the sense node. Exposures may be performed in a rolling shutter mode where the exposure time is 4 rows. This mode involves exposing the first row of pixels, and then with a delay of one row, the second row is exposed. The third row is exposed with a delay of 2 rows and the fourth with a delay of 3 rows. When the fifth row is exposed (delay of 4 rows), the first row is no longer exposed and is read out. When the sixth row is exposed, the second row is read out, and so forth. Thus, each of the pixels per row sees a constant time of 4 rows of exposure.
In U.S. Patent Application Publication US2004/0263648, a method and apparatus are disclosed that identify and compensate for dark current effect in an imaging device. The method includes capturing and storing both dark and white reference images.
U.S. Patent Application Publication US2004/0095488 provides a method for testing pixels by exposing them to known quantities of radiation to correct for defective pixels.